A predictive thermodynamic model for the bioreduction of acetophenone to phenethyl alcohol using resting cells of Saccharomyces cerevisiae

Equilibrium conversions were observed in the range of 60.2-76.0% with diffe rent initial compositions of reaction media for the bioreduction of acetoph enone using resting cells of Saccharomyces cerevisiae in aqueous solutions at 30 degrees C. The reduction of acetophenone in the cells under anaerobic conditions is considered to be coupled with the oxidation of ethanol to ac etate in the cytoplasm. A biphasic thermodynamic model is proposed which in cludes a nonuniform distribution of reagents across the cell membrane, a tr ansmembrane pH gradient, ideal and nonideal solution models, and a basic re action stoichiometry (ACP + 1/2EtOH + 1/2H(2)O tt PEA + 1/2Ac(-) + 1/2H(+)) . The intracellular activity coefficients were based on the Lewis-Randall r ule for acetophenone, phenethyl alcohol, and H2O and Henry's law for ethano l, acetate anion, and H+. The overall standard Gibbs free energy was estima ted to be -0.11 kcal/mol at a pH 7, 25 degrees C, and 1 atm. The intracellu lar thermodynamic activity coefficients of acetophenone and phenethyl alcoh ol were predicted to be 471.2 and 866.4, respectively, using the measured i nitial distribution coefficients and calculated extracellular activity coef ficients. The model reflected a zero Gibbs free energy change at calculated conversions within 4% of the measured equilibrium conversions. The analysi s verified the effect of the concentration ratio of the substrate acetophen one to the cosubstrate ethanol on the conversion efficiency and suggested t hat the intracellular pH and the pH gradient across the cell transmembrane significantly affect the predicted equilibrium conversion. The intracellula r pH of resting, viable cells of Bakers' yeast at the bioconversion conditi ons was determined experimentally to be 5.77. (C) 1999 John Wiley & Sons, I nc.